Method and apparatus for drying air

ABSTRACT

There is described a method of drying air with the aid of a rotor (6) which rotates continuously in a housing (2) and which incorporates moisture-sorbent means. The housing (2) has mounted therein a casing (12) which is located on the low-pressure side of the rotor and provided with heat-emitting devices, for instance electrical elements (15), for delivering radiant heat directly to the rotor interior. The heat required for regeneration is therewith delivered at high temperature, which increases the heat yield. The part of the rotor (7) which is normally the most difficult to regenerate is therewith heated to a temperature of 250°-300° C. Since the volume of regeneration air is smaller than normal, the air is heated to a temperature which is much higher than normal in all parts of the rotor, so as to result in effective removal of the moisture present. The invention also relates to an air dryer by means of which the method can be put into effect.

TECHNICAL FIELD

The present invention relates to a method of drying air with the aid ofa preferably continuously rotating rotor which is mounted in a housingprovided with an inlet. Air which is pressurized by means of a fanpasses through a rotor part, in which moisture present in the air isextracted therefrom by a moisture-sorbent located in the rotor part. Hotair is conducted through another part of the rotor such as toregenerate, the rotor part while taking-up moisture and is dischargedfrom the housing. The air treated in the rotor departs through the rotoroutlet.

The invention also relates to an air-dryer or dehumidifier, and moreparticularly, but not exclusively, to an air-dryer including a housingprovided with an inlet and outlet, a rotor which is journaled in thehousing and which incorporates passages or a bed which contains amoisture-sorbent (e.g., silica-gel crystals), a means for driving therotor, a fan for pressurizing air supplied to the inlet, such that theair will flow through at least one part of the rotor and, subsequent tomoisture-extraction, will depart through the outlet, means for heatingthe regeneration air passing through a further part of the rotor, andheat-emitting devices mounted adjacent the rotor on its low-pressureside and which are operative to deliver radiant heat axially onto themoisture-sorbent disposed in the rotor passages or bed.

BACKGROUND PRIOR ART

Known air-dryers of the kind which operate in accordance with the above,in which the rotor normally rotates at a speed of about 25 rev./h, donot effectively utilize the supplied energy and therefore incur highoperation costs. Such air-dryers are also large and heavy, and thereforeexpensive.

The air-dryers are also relatively complicated structurally, andconsequently are expensive as a whole and difficult to dismantle, forinstance for servicing and maintenance purposes.

Present-day air-drying or dehumidifying systems are also constructedfrom materials which render the useful life of such systemsunsatisfactory.

The unsatisfactory efficiency of present-day air drying apparatus isdue, inter alia, to the fact that the regenerating battery is located atsome considerable distance from the rotor. The regeneration air--whichnormally has a temperature of 110°-120° C.--is therefore incapable ofextracting moisture effectively, particularly moisture which is capturedin the nooks and crannies of those rotor-parts which are located in theregion of the low-pressure side of the rotor, where the moisture is mostdifficult to extract.

Also known to the art are air-dryers or dehumidifiers of the kind whichinclude a moisture-adsorbing rotor provided with drive means, two fansdriven by a common motor, of which one fan is intended for process airand the other fan intended for regeneration air, and filters forfiltering the two air-flows.

This known drying apparatus is divided into five sections, placed oneabove the other. The requisite electrical equipment is housed in the lidor cover of the dryer, together with the rotor drive-motor The two fans,the motor which is common to the fans, and the filters are housed in thesecond and third sections respectively. The next lowest section houses adistribution chamber operative to distribute process-air andregeneration-air, together with a heating battery. The actual rotoritself is housed in the bottom section, with the rotor in a horizontal,i.e. with a vertical axis. Subsequent to regenerating the rotor in thelowermost section of the dryer, the wet air and the dry air are led awayfrom said section in mutually different directions.

This apparatus is also ineffective, since that part of the rotor wherethe moisture is captured most effectively, i.e. is the most difficult toextract, is not regenerated at a sufficiently high level of temperature.

Other examples of the present state of this art are to be found inGB-A-2165465 (Munter Rotair) and SE-B-429301 (Munters).

OBJECT OF THE PRESENT INVENTION

The object of the present invention is to eliminate the aforesaiddrawbacks of the conventional known air-drying methods and apparatus,and to improve the heat-yield of the process by recovering and utilizingthe energy delivered to the regenerating air in a more effective manner.

SUMMARY OF THE INVENTION

This and other objects are fulfilled by a method of the aforesaid kindwhich is mainly characterized by bringing the regeneration air into theclose proximity of heat-emitting devices which are placed on thelow-pressure side of the rotor and which are operative to deliverradiant heat axially and directly onto moisture-adsorbing means ormoisture-absorbing means located in the rotor interior.

By mounting the heat-emitting devices in close proximity with the rotor,the heat required to regenerate the rotor can be given a hightemperature, which therewith increases the heat-yield in, inter alia,the rotor passageways, i.e. eliminates heat losses.

Furthermore, by utilizing radiant heat to regenerate the rotor, thatpart of the rotor which is normally most difficult to regenerate will beheated satisfactorily.

In accordance with one preferred embodiment of the invention, heatstored in the rotor is used to preheat a captured air-volume, which isdeflected and constitutes regeneration air. Thus, preheated air enters aregeneration battery which covers a given part of the rotor. Theregeneration battery is primarily operative to radiate thermal energydirectly into the rotor interior. This purely radiation-heat providesthe highest yield; the captured and deflected, preheated air-flow, whichis further heated during its passage through the casing, is alsorequired, however, to transport away moisture from parts of the rotorwhich are located nearer the high-pressure side of the rotor and whichare therefore not reached by radiant heat from the regeneration batteryequally as effectively.

In combination, the radiant heat of high energy-level is delivered tothe rotor where the greatest difficulties exist in removing moisturetaken-up by the sorbent, while energy of a lower level is utilized whereremoval of the water is more simple.

Thus, the inventive method enables the energy available to be utilizedmore rationally, and therewith increases the efficiency of the dryingair process.

In practice, it is preferred that the air preheated during its passagethrough the rotor is deflected by the casing and heated by theheat-emitting devices to a temperature which, together with the radiatedheat, Will bring the rotor-material situated adjacent the casing to atemperature higher than 200° C., preferably above 250° C.

This will ensure that the thermal energy delivered will be usedoptimally.

It is also preferred that 15-35%, preferably about 20%, of the airpressurized by the fan and passing through the rotor is captured by thecasing and utilized as regeneration air.

Trials have shown that the highest efficiency afforded by the inventivemethod is achieved when about 1/5th-1/6th of the pressurized air isrecycled through the rotor as regeneration air. Since this volume of airis smaller than normal, the regeneration air will have a highertemperature in all parts of the rotor and is therefore able to removemoisture in a more effective manner than known constructions of thiskind.

The rotors of conventional air-drying apparatus normally rotate at aspeed of from 10 to 12 revolutions per hour (rph). Because the presentinvention enables the thermal energy delivered to be recovered moreefficiently, it has been found possible in practice to increase therotor speed in relation to what has previously been considered anoptimum speed, thereby further improving the efficiency of the apparatusand also enabling the apparatus to be of lighter, and therewith lessexpensive construction.

When practicing the method according to the present invention, the rotorspeed will preferably lie within the range of 15-30 rph, wherein a rotorspeed of about 25 rph has been found to provide the best result.

In practice, it is also preferred that the regeneration air recycledthrough the rotor is captured by a further casing located opposite thefirst mentioned casing on the pressure-side of the rotor, from which thewet regeneration air is discharged from the housing, for instancethrough a discharge conduit.

As before mentioned, the invention also relates to an air-dryingapparatus. This apparatus is mainly characterized by an air dryerincluding a housing provided with an inlet and an outlet, a rotor whichis journaled in the housing and which incorporates passages or a bedwhich contains a moisture-sorbent (e.g., silica-gel crystals), a meansfor driving the rotor, a fan for pressurizing air supplied to the inlet,such that the air will flow through at least one part of the rotor and,subsequent to moisture-extraction, will depart through the outlet, meansfor heating the regeneration air passing through a further part of therotor, and heat-emitting devices mounted adjacent the rotor on itslow-pressure side and which are operative to deliver radiant heataxially onto the moisture-sorbent disposed in the rotor passages or bed.

An air-dryer constructed in accordance with the invention willdemoisturize air dynamically, in a continuous process, such as to enablea constant moisture-climate to be achieved.

Various drying agents can be used to dry the air, for instance absorbentor adsorbent agents. Hygroscopic salt, e.g. lithium chloride, is anexample of a suitable absorbent in this context. The molecular structureof the drying agent changes as it takes-up moisture. In the case oflarge quantities of moisture, some drying agents or desiccants may beconverted to a liquid form. A drying agent of this nature is used toimpregnate the rotor of a rotary dehumidifier or air-dryer.

Sorbent drying agents include materials which do not change physicallyor chemically when taking-up moisture. Aluminium oxide is an example ofone material which belongs to this category and which is used at highpH-values and high relative humidity.

Molecular sieves form a category of substances which are highlyeffective in the present context, particularly when wishing to achieveextremely low air moisture contents. Molecular sieves, however, requirehigh energy input and high regeneration temperatures, and consequentlythe method proposed in accordance with the invention is particularlysuitable in this regard.

Silica-gel is a crystalline substance capable of taking-up largequantities of moisture, e.g. quantities coresponding about 40% of itsown weight. Various types of silica gel are available, for differentareas of use.

The last mentioned sorption agents or agents closely related thereto,i.e. silica gel or a molecular sieve, are used when practicing thepresent invention. It will be understood, however, that the inventioncan be applied with all available types of desiccants.

The invention can also be practiced with various types of rotors. Apreferred rotor is one which includes a large number of passages capableof accommodating a moisture-sorbent, e.g. silica gel. This agent mayalso consist of a desiccant bed.

The casing which captures and deflects part of the dehumidified air inthe rotor, and which therewith accommodates the heat-emitting devices,is preferably made of a highly reflective material, such as to reflectback radiation which is directed away from the rotor. Aluminium or anappropriate alloy thereof is a suitable casing material in this respect.

Other materials may also be used, however, and the casing may comprise arelatively inexpensive metal or plastics material provided with areflective coating.

The invention will now be described in more detail with reference to anexemplifying embodiment thereof illustrated in the accompany schematicdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an air-dryer or dehumidifier constructedin accordance with the invention, the housing of the air-dryer beingassumed to be transparent for the sake of illustration, so as to enablethe various components of the air-dryer to be shown.

FIG. 2 is a vertical section through an air-dryer according to FIG. 1.

FIG. 3 is an exploded, perspective view of part of the rotor andillustrates casings which co-act with the rotor and which are locatedrespectively on the high-pressure and the low-pressure side thereof.

FIG. 4 is a sectional view of an embodiment alternative to the FIG. 2embodiment.

FIG. 5 is a schematic and greatly enlarged view of a silica-gel crystaland illustrates theoretically the events taking place when regeneratingthe crystal and thus also the rotor.

DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to FIGS. 1 and 2, an air dryer 1 includes a housing 2provided with an inlet 3 and an outlet 4.

Mounted in the housing 2 is a partition wall 5. The partition wall hasprovided therein a circular hole which accommodates a rotor 6, thejournals of which are not shown in the drawings. The rotor is driven byan electric motor 7, via a belt 8, and is provided with a suitable seal9 which seals the rotor against the partition 5.

The rotor 6 incorporates a large number of passages 6a, in which dryingagent, for instance silica gel, a molecular sieve or the like isaccommodated, therewith permitting practically unlimited regeneration ofthe rotor to be effected.

Located adjacent the inlet 3 is a centrifugal fan 10 which is operativeto draw moist air into the inlet 3. The fan functions to create anoverpressure in that part 2a of the housing which is located adjacentthe partitioning wall 5. The air drawn-in and pressurized by the fan 10will pass through the rotor 6 and is dehumidified and preheated therein.The major part of this air-flow enters a chamber 2b located on the otherside of the partitioning wall and departs through the outlet 4, and issubsequently used for a useful purpose, for instance dry-air storage orfor material-drying purposes in connection with some other process.

A minor part of the air passing through the rotor 6, e.g. one-fifth ofthe incoming air-flow, is captured by a casing 12, which is made ofaluminium or some like material and mounted closely adjacent to therotor 6 on the low-pressure side thereof The casing 12 has approximatelythe shape of the sector of a circle, with the exception of a partcorresponding to the rotor shaft.

The casing 12 is divided into two chambers 12b and 12c by means of adividing wall 12a. A further, similar casing 13 corresponding to thechamber 12c is located on the high-pressure side of the rotor, closelyadjacent to the rotor and is sealed there against in the housing part2a.

As mentioned, part of the air which flows through the rotor is capturedby the casing part 12b. This captured air is deflected through 90° andpasses into the chamber 12c, through an opening in the wall 12a.Heat-emitting devices in the form of electrical elements 15 are mountedin the chamber 12c.

The electrical elements 15 are mounted closely adjacent the rotor 6 andare intended to deliver strong radiant-heat to parts of the rotor lyingclose to the casing 12. The drive arrangements 7, 8 are intended todrive the rotor counterclockwise in the housing 2.

In addition to regeneration of the sorbent-substance by the heatedair-flow deflected by the casing 12 and recycled through the rotor, thesorption agent is also regenerated by the radiant heat delivered by theheat-emitting electrical elements 15.

Thus, high-temperature thermal energy will radiate directly into therotor, wherein those rotor-parts which lie adjacent the casing will beheated to a temperature above 250° C., possibly above 300° C.

That part of the rotor 6 where moisture is most difficult to reach andtherefore most difficult to extract will thus undergo the most effectiveregeneration, mainly due to the radiant energy delivered by theelectrical elements 15.

The air-flow which is preheated during its passage through the rotor anddeflected and recycled therethrough, subsequent to being heated by theelectrical elements 15, is also necessary for regeneration purposes,however This air-flow thus regenerates effectively primarily those partsof the rotor which are located close to its high-pressure side, andwhich are therewith not so difficult to extract as the rotor partslocated adjacent the casing 12 and where the radiant heat from theelectrical elements 15 has only a limited effect.

FIG. 5 is a schematic, greatly enlarged view of a silica-gel crystal,and shows the principle configuration of such a crystal and indicates inprinciple the events that occur when water is regenerated from a crystaldesiccant. The crystal is designated by reference numeral 30.

As illustrated in FIG. 5, the crystal has a large number of passages30a, and a single gram of crystal of the aforesaid kind can present aspecific surface area of about 500 m². The figure also illustrates awater molecule H--O--H located in one such passage, while the arrow Pindicates the passage of air through the crystal.

The figure gives an indication that the higher the temperature thehigher the driving-force by means of which water is removed from thecrystal during regeneration and the easier it then becomes to take-upwater in the drying process.

An understanding of this mechanism will provide a theoreticalexplanation of the advantages afforded by the invention.

The moist regeneration-air is collected in the casing 13 and dischargedfrom the housing 2 through a hose 17.

A support structure for supporting the rotor-journals is mounted in thepartition wall 5 and includes, inter alia, vertical U-bars 20, one oneach side of the wall 5, bolts 21, a plate 22 which carries the rotorjournals (not shown), and bolts 23 which secure the plate 22.

The air-flow generated by the fan 10 is deflected by a curved plate 24.

In the case of the illustrated embodiment, the housing 2 may measure 0.4m×0.4 m×0.4 m and the rotor may be rotated at a speed of 25 rph.

In air-dryers of this kind, the volume of dry air exiting through theoutlet 4 may be in the order of 200 m³ /hour. In the present case,approximately one-fourth of this air volume, or more, is captured by thecasing 12 and recycled through the rotor 2 for use as regeneration air.

Trials have shown that when the rotor speed is increased from 10 to 25rev./hour, the capacity of the air dryer will also increase by about20%, all other things being equal. Further increase in the rotor speed,however, will result in reduced capacity.

Consequently, the rotor speed should lie within the range of 15-30rev./hour and preferably about 25 rev./hour.

An excessive decrease in the volume of regeneration air will result in adecrease in the efficiency of the air dryer, since because theradiant-energy effect cannot be properly utilized, the moisture cannotbe carried away effectively. The invention, however, enables smallerquantities of air to be used, and therewith higher temperature levels,resulting in effective regeneration.

Practical trials have showns that in the case of a dryer of theaforesaid capacity value, i.e. a dryer which will produce 200 m³ dry airper hour, the optimal power consumption is 1.6 to 1.7 kW. In the case ofa dryer of this optimum construction, the rotor speed should be 25rev./hour and the volume of air used for regeneration should be 40 m³/hour.

An air dryer which operates on the basis of these values will providethe greatest possible regeneration effect per unit of energy input.

The aforesaid values apply with a rotor depth of 100 mm. Although agreater rotor depth, e.g. a rotor depth of 200 mm, will increase thecapacity of the dryer by about 10%, the dryer in general will havelarger dimensions and will be much more expensive to manufacture.

It has been found in practice that a rotor depth of 100 mm isappropriate for air dryers which deliver up to 600 m³ of dry air perhour. A rotor depth of 200 mm is appropriate for dry-air volumes abovethis level.

The afore-mentioned values shall be considered as guides capable ofillustrating variations contingent on other parameters applied with thedryer.

FIG. 4 illustrates an embodiment which has been modified in relation toFIGS. 1-3, insofar none of the original flow through the rotor 6 of themodified embodiment is captured and deflected on the low-pressure sideof the rotor.

In the case of the FIG. 4 embodiment, regeneration air is, instead,supplied directly through a conduit 18 connected to a modified casing12'. The regeneration flow may optionally be preheated by means notshown, and further heated during its passage through the casing, inwhich heat-emitting devices in the form of electrical elements 15 aremounted and arranged in the same manner as with the afore-describedembodiment, i.e. the electrical elements 15 are mounted closely adjacentthe rotor 6 and are intended to deliver strong radiant heat axially ontothe rotor parts located adjacent the casing 12'.

The heat-emitting devices have the same function as the heat-emittingdevices described with reference to the earlier embodiment.

I claim:
 1. A method of drying air with a rotating rotor mounted in a housing provided with an inlet and first and second outlets, wherein air entering via said inlet is pressurized by a fan and passed through a first part of the rotor and moisture present in the air is taken-up by moisture-sorbent means disposed in said rotor, and wherein a first portion of said air exits said housing through said first outlet and a second portion of said air is heated and passed through a second part of the rotor for regenerating said rotor part while taking-up moisture, and is discharged from the housing through said second outlet, said method comprising the step of causing the regeneration air to flow in heat transfer relationship with heat-emitting devices which are mounted adjacent the rotor on a low-pressure side thereof and which are operative to direct radiant heat axially onto the moisture-absorbent means located in the interior of the rotor, wherein the regeneration air is heated to a temperature, which together with the radiant heat emitted, will bring the rotor material adjacent said casing to a temperature above 200° C.
 2. A method according to claim 1 wherein said rotor is rotated at a speed of 15-30 rev./hour.
 3. A method according to claim 1, wherein the rotor material adjacent said casing is above 250° C.
 4. A method according to claim 1, further comprising the step of passing the regeneration air to a casing which is located on the low-pressure side of the rotor and which accommodates the heat-emitting devices.
 5. A method according to claim 4, in which a minor part of the pressurized air is captured on the low-pressure side of the rotor and, subsequent to being heated, is recycled through said second rotor-part as regeneration air, said method further comprising the step of capturing and deflecting said regeneration air by said casing prior to said air passing the heat-emitting devices.
 6. A method according to claim 5, wherein said casing captures 15-35% of the dry-air volume produced, and utilizing said captured volume as regeneration air.
 7. A method according to claim 6, wherein said casing captures 20% of the dry-air volume produced, and utilizing said captured volume as regeneration air.
 8. An air dryer comprising:a housing provided with an inlet and an outlet; a rotor which is disposed in the housing such that air supplied from the inlet will flow through (at least) one part of the rotor comprising passages or a bed which contains a moisture-sorbent, wherein moisture in the air will be extracted; means for driving the rotor; a fan in said housing for pressurizing air supplied via the inlet, such that the air will flow through said (at least) one part of the rotor and, subsequent to said moisture-extraction, a first portion of the air will depart through the outlet; passage means communicating with the rotor for receiving a second portion of the air comprising means for heating said second portion of the air which constitutes the regeneration air passing through a further part of the rotor; including heat-emitting devices mounted adjacent the rotor within the passage means on its low-pressure side in a casing and which are operative to deliver radiant heat axially onto the moisture-sorbent disposed in the rotor passage or bed together with the heated regeneration air and outlet means from the housing for receiving the regenerated moisture laden air.
 9. An air dryer according to claim 8, wherein the rotor is mounted in a recess in a partition wall mounted in the housing and is provided with sealing means for sealing the rotor in relation to said wall; and said casing is sealed in relation to said rotor.
 10. An air dryer according to claim 8, wherein said casing comprises a material which will reflect radiant heat back into the rotor.
 11. An air dryer according to claims 8 or 10, wherein the first casing is approximately of circle-sector configuration with a part corresponding to the rotor shaft, and covers approximately one-fourth of the rotor surface; and a second casing is located opposite the first casing on the high-pressure side of the rotor, said second casing functioning to catch the moisture regeneration air and to conduct said air from the housing through a hose.
 12. An air dryer according to claim 10, wherein said material is aluminum. 